The success of cisplatin and its derivatives as anticancer agents has stimulated the development of metal-based compounds, including that of platinum and gold, for anticancer treatment [L. Kelland, Nat. Rev. Cancer 2007, 7, 573; C. F. Shaw III, Chem. Rev. 1999, 99, 2589]. In this context, extensive investigations on the biological properties of platinum(II), gold(I) and gold(III) have been reported. However, the development of the metal-based, particularly gold(III), as potential anti-cancer agents has been hampered by their poor stability in solution [M. Coronnello, E. Mini, B. Caciagli, M. A. Cinellu, A. Bindoli, C. Gabbiani, L. Messori, J. Med. Chem. 2005, 48, 6761]. To our knowledge, very few cytotoxic gold(III) compounds such as [Au(bipyc-H)(OH)][PF6] (bipyc-H=deprotonated 6-(1,1-dimethylbenzyl)-2,2′-bipyridine), [Au(dmamp)Cl2] [dmamp=2-(dimethylaminomethyl)phenyl], and gold(III) tetraarylporphyrins [C.-M. Che, R. W.-Y. Sun, W.-Y. Yu, C.-B. Ko, N. Zhu, H. Sun, Chem. Commun. 2003, 1718], have been reported to have significant stability.
The synthesis and photophysical properties of various cyclometalated gold(III) N-heterocyclic complexes have been reported by Yam et al [J. Am. Chem. Soc. 2009, 131, 9076; US 2009/0278453 A1]. Yet, the biological properties, notably the anti-cancer properties, of these complexes are completely unknown in the literature.
Cyclometalated platinum(II) complexes containing π-aromatic ligands have long been known to be metallointercalators for double-stranded DNA as the planar metal complex cations can insert between DNA base pairs through ligand-ligand π-π stacking interactions [Chan, C. W.; Cheng, L. K.; Che, C. M. Coord. Chem. Rev. 1994, 132, 87]. Extensive studies have revealed that [PtII(terpy)(X)]+ (terpy=2,2′:6′,2″-terpyridine, X=chloride, 2-aminoethanethiolate, ethyl 2-mercaptoacetate, 2-hydroxyethanethiolate or cysteine), [PtII(N^N)(en)]2+ (N^N=1,10-phenanthroline or 2,2′-bipyridine; en=ethylenediamine) and [PtII(CNN)(X)]+ (CNN=6-phenyl-2,2′-bipyridine, X=pyridine, 4-aminopyridine or N,N′-bis(isonicotinyl)-1,6-hexane-diamine) can intercalate DNA and display cytotoxic activities [(a) Howe-Grant, M.; Lippard, S. J. Biochemistry 1979, 18, 5762; (b) Lowe, G; Droz, A. S.; Vilaivan, T.; Weaver, G. W.; Park, J. J.; Pratt, J. M.; Tweedale, L.; Kelland, L. R. J. Med. Chem. 1999, 42, 3167]. Platinum(II) complexes have other potential biological applications because they are usually kinetically stable, soluble in water and do not form insoluble hydrated oxides under physicological conditions. The tendency of square-planar platinum(II) complexes to form one-dimensional columnar stacks in their crystal structures and the aromaticity and size of chelating aromatic ligand such as terpy, N^N or CNN all contribute to the ability of platinum(II) complexes to bind to DNA by intercalation.
The chemistry of N-heterocyclic carbene (NHC) has long been confined to metal coordination complexes derived from azolium compounds, which was started by Öfele and Wanzlick in 1968. In 1991, Arduengo successfully synthesized stable free NHCs, which had subsequently been used as ligands for transition metal complexes [Arduengo, A. J. III; Kline, M.; Calabrese, J. C.; Davidson, F. J. Am. Chem. Soc. 1991, 113, 9704.]. Since then, many transition metal carbene complexes have been reported [Herrmann, W. A.; Köcher, C. Angew. Chem. Int. Ed. Engl. 1998, 36, 2162. (b) Bourissou, D.; Olivier, G; Francois, P. G; Bertrand, G. Chem. Rev. 2000, 100, 39]. A leading motive is the advantage of NHC as ligand in the development of organometallic catalysts, whereas NHC ligands extend the scope of applications reached by phosphanes (functionalized, chiral, water-soluble, and immobilized derivatives). Metal-NHC complexes are usually stable to heat, air, and moisture [Herrmann, W. A.; Goossen, L. J.; Spigler, M. Organometallics. 1998, 17, 2162], and the coordination of carbene ligand to metal ion can be performed under simple and mild conditions. This feature stimulates a surge of interest, and many transition metal complexes containing NHC ligands derived from imidazolium ions have been synthesized [Herrmann, W. A.; Köcher, C. Angew. Chem. Int. Ed. Engl. 1997, 96, 2162]. Metal-N-heterocyclic carbene complexes have been used as catalysts for a spectrum of catalytic reactions, including Heck, Suzuki, and Kumada coupling reactions, alkene metathesis, and hydrosilylation [Huang, J.; Nolan, S. P. J. Am. Chem. Soc. 1999, 121, 9889]. In general, NHC ligands are accessible and their strong σ-donating character resemble to that of phosphine ligands [Herrmann, W. A.; Köcher, C. Angew. Chem. Int. Ed. Engl. 1997, 96, 2162]. In literature, however, there are few PtII-NHC complexes [(a) Unger, Y.; Zeller, A.; Ahrens, S.; Strassner, T. Chem. Commun. 2008, 3263. (b) Liu, Q. X.; Xu, F. B.; Li, Q. S.; Song, H. B.; Zhang, Z. Z. Organometallics 2004, 23, 610. (c) Fantasia, S.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Organometallics, 2007, 26, 3286. (d) Fantasia, S.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Organometallics, 2007, 26, 5880. (e) Liu, Q. X.; Song, H. B.; Xu, F. B.; Li, Q. S.; Zeng, X. S.; Leng, X. B.; Zhang, Z. Z. Polyhedron 2003, 22, 1515. (f) Quezada, C. A.; Garrison, J. C.; Tessier, C. A.; Youngs, W. J. J. Organomet. Chem., 2003, 671, 183.], and none of reported examples contain chelating cyclometalated ligand such as 6-phenyl-2,2′-bipyridine.